Refactor search code to reduce function size. (#11991)

Splits the search code into a few logical functions instead of a single
unreadable function.

There are also a few additional changes for readability.

After refactoring it was clear to see there were some unused and
unnecessary variables, which were simplified.

synapse/storage/databases/main/search.py

@@ -28,6 +28,7 @@ from synapse.storage.database import (
 )
 from synapse.storage.databases.main.events_worker import EventRedactBehaviour
 from synapse.storage.engines import PostgresEngine, Sqlite3Engine
+from synapse.types import JsonDict
 
 if TYPE_CHECKING:
     from synapse.server import HomeServer
@@ -381,17 +382,19 @@ class SearchStore(SearchBackgroundUpdateStore):
     ):
         super().__init__(database, db_conn, hs)
 
-    async def search_msgs(self, room_ids, search_term, keys):
+    async def search_msgs(
+        self, room_ids: Collection[str], search_term: str, keys: Iterable[str]
+    ) -> JsonDict:
         """Performs a full text search over events with given keys.
 
         Args:
-            room_ids (list): List of room ids to search in
-            search_term (str): Search term to search for
-            keys (list): List of keys to search in, currently supports
+            room_ids: List of room ids to search in
+            search_term: Search term to search for
+            keys: List of keys to search in, currently supports
                 "content.body", "content.name", "content.topic"
 
         Returns:
-            list of dicts
+            Dictionary of results
         """
         clauses = []
 
@@ -499,10 +502,10 @@ class SearchStore(SearchBackgroundUpdateStore):
         self,
         room_ids: Collection[str],
         search_term: str,
-        keys: List[str],
+        keys: Iterable[str],
         limit,
         pagination_token: Optional[str] = None,
-    ) -> List[dict]:
+    ) -> JsonDict:
         """Performs a full text search over events with given keys.
 
         Args: